The present invention relates generally to switch mode power supply (SMPS) systems. More particularly, embodiments of the invention relate to methods and circuits for controlling and improving the power factor of SMPS systems.
Switch mode power supply (SMPS) systems have many advantages over conventional linear regulated power supplies. These advantages include smaller volume, better stability, and higher power efficiency. As a result, SMPS has found widespread applications, such as televisions, set-top boxes, and video recorders, portable telephone chargers, personal digital assistants (PDAs), and even certain automated tooth brushes. In recent years, as light emitting diode (LED) technologies are becoming more mature, SMPS systems are widely used as drivers for LED devices, including as drivers in white-light bulb replacement applications.
However, conventional SMPS for LED lighting systems suffer from many limitations. As an example, when an SMPS is used to drive an LED, it generally needs to provide a constant output current in order to maintain stable brightness of the LED. When used with different AC power sources, for example, 110V vs. 220V, conventional LED lighting devices often need to be customized for the local power sources. Often a different control circuit is required, or an additional power source selection circuit is needed.
Moreover, unlike conventional incandescent light bulbs, LED lighting devices do not behave like a purely resistive load in an AC circuit. Therefore, conventional LED light bulbs often do not provide desirable efficiency in the utilization of the AC power supply. The efficiency of an AC electric power system can be measured by “power factor”. As used herein, the power factor of an AC electric power system is defined as the ratio of the real power flowing to the load to the apparent power in the circuit. Real power is the capacity of the circuit for performing work in a particular time. Apparent power is the product of the current and voltage of the circuit. A recent U.S. energy efficiency standard requires an LED with greater than a 5 W power rating to have a power factor no lower than 0.7. A European standard requires an LED with more than 25 W to have a power factor higher than 0.94. Conventional devices often cannot meet these standards.
Furthermore, in home or hotel lighting applications, traditional incandescent light bulbs are often used with a silicon dimmer device to adjust the brightness. Conventional LED lighting devices often cannot accommodate a dimmer circuit, and cannot provide the brightness adjustment capability.
Therefore, more efficient and cost-effective techniques for improving power supply systems for LED lighting applications are highly desired.